This invention relates to non-destructive methods of investigating the surface properties of a body of ferromagnetic material and to apparatus therefore. In particular, it is concerned with the determination of the surface profile of the inside of pipelines constructed from materials such as iron or steel.
In the past bodies of ferromagnetic material have been inspected by methods such as the flux leakage method, in which the metal is magnetised in a direction parallel to its surface. At defects, or where regions of the metal body are thinner, some magnetic flux passes into the air and may be detected by sensors, thus giving an indication of the presence of faults.
Another type of magnetic inspection system includes the body to be inspected within a magnetic circuit which incorporates an air gap. Measurement of the field within the gap enables information about the nature of the inspected component to be derived. This system has the advantage that it does not usually require such high intensity magnetic fields as the flux leakage method, but is has the drawback that it is sensitive to variations in, for example, the diameter of the pipeline under test, or other factors affecting the geometry of the magnetic circuit, as well as to surface defects.
Work has been carried out to devise a new improved method of non-destructive testing of bodies of ferromagnetic material.